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Spinning into control

20 June 2013

Researchers at the University of Southampton have devised a simple current sensor using birefringent optical microfibres and exploiting the Faraday effect. Their method will not require any complicated manufacturing processes for the microfibres, or any post-fabrication treatment. As such, the technique is robust to any manufacturing imperfections and can provide detailed real-time monitoring.

Left and right

The Faraday effect, observed by Michael Faraday in 1845, was the first phenomenon to imply a relationship between light and electromagnetism. It occurs in optically transparent materials in the presence of a magnetic field and, in the simplest sense, causes the linear polarisation direction of the incident wave to rotate around the axis of propagation.

The effect is most easily understood by considering circularly polarised light. A linearly polarised beam can be described as a superposition of two circularly polarised beams rotating in opposite directions, left and right polarised. The angle of the linear polarisation will be determined by the phase between the two circular components. If this phase difference is changed, then the polarisation will rotate.

As the linearly polarised beam propagates in the medium, the motion of charged particles will be influenced by the beam itself and by the applied field. The right-hand polarised component will have the opposite effect from the left-hand polarised component on the local magnetic field, created by the motion of the charged particles. This will either reinforce or oppose the external field and the phases of the two components will shift, causing rotation of the linear polarisation. The rate of polarisation rotation is, in fact, proportional to the applied field.

Given this dependence on external field, the effect can be used to sense or detect current very sensitively, by coiling an optical fibre around a current carrying cable, which will generate the magnetic field that can be measured. However, other effects are known to hinder this method. One of the main problems is the intrinsic linear birefringence of the fibre, produced by imperfections and the strain caused by coiling.

Pushing periodically

Linear birefringence is an effect where a medium has different refractive index depending on the light’s linear polarisation. This difference also causes rotation, which can interfere with any true Faraday rotation. Furthermore, the linear birefringence will cause a reversal in rotation direction periodically and, depending on the measurement position, it is possible for no rotation to be observed.

To address these problems, the team have developed a system where they locally alter the linear birefringence of the fibre using a femto-second laser at optimum points in the coil’s path.This can cause the periodic linear rotation to jump its phase by π, and the rotation will continue in a consistent direction.

This variable birefringence could be built in to the fibre at the fabrication stage, but that can be expensive and not necessarily accurate or reliable. George Chen, one of the authors of the research explained that “compared to spinning the microfibre during tapering, which requires careful control of the spin rate and intrinsic birefringence, this post-fabrication approach has reduced design complexity due to the simple fabrication and treatment procedures, and the ability to monitor the improvement in current responsivity in real-time”.

Curing and bubbles

The next step for the group is to prove their concept - to “study and quantify the tolerance of the birefringence treatment”. Chen also explained that “the optical loss and robustness of the fabricated samples are important factors that influence the long-term performance of these sensors - work still needs to be done to minimize the amount of air bubbles in the polymer material, and to optimize the curing conditions”.

The Southampton group is also pursuing other technologies associated with optical fibre-based sensors. “I am a part of the research group led by Dr. Trevor Newson, which investigates a variety of optical fibre sensors for sensing applications including temperature, strain, acceleration and acoustic”, said Chen. “My long-term goals”, he continued, “are to develop novel point-based sensors that can offer high sensitivity and speed of response, in compact packages that are potentially available at low cost. My colleagues aim to deliver long-range, high-resolution distributed sensing systems that can be used to monitor structures and chemical processes in unfriendly environments such as pressure vessels, brick-lined reactors ovens and driers”.